Most modern turbocharged vehicle engines include some form of factory-fitted blow-off or bypass valve in the intake tract, the purpose of which is to open during throttle closure to provide a relief path from a diversion aperture or pressurised air that would otherwise cause significant pressure spikes, resulting in damage or reduced life-span of the turbocharger, and also an unpleasant fluttering noise that is deemed unacceptable in a road-going passenger car.
There is also a potential performance improvement, in that without a relief path, rapid throttle closure and the resulting pressure spike can rapidly slow the turbo compressor, leading to a longer delay in returning to peak boost when the throttle is re-opened (i.e. increased turbo lag). Similarly, allowing the bypass valve to relieve too much pressure can also have an adverse effect on turbo lag—evacuating the entire volume of the intake tract means that despite maintaining a higher compressor speed over the short term, when the throttle is re-opened the intake tract must be re-pressurised which causes an increase in lag.
Many factory-fitted bypass valves perform additional tasks, such as limiting boost pressure for engine protection in the event of higher-than-normal boost pressure being detected, and momentary power reduction when required for traction control or during a gearshift in the case of automatic transmissions.
Factory or OEM fitted bypass valves. Until recently, most factory-fitted bypass valves have been located somewhere on the engine's intake tract downstream of the turbo compressor, usually being mounted either by hose or flange connection. The vented air from the bypass valve is then directed back to the turbo compressor intake via another hose or duct thus forming a bypass loop around the compressor.
Newer implementations include a mounting flange built directly into the turbo's compressor cover, which includes separate paths for both the incoming pressure and the vented air to be recirculated in the one flange.
The factory bypass valve can be fitted to this type of flange, and is a direct-actuated solenoid type, that is controlled by a signal from the car's Engine Control Unit (ECU). This type of valve features an electric solenoid coil which has a plunger, on which plunger is mounted a valve member to open or close the diversion aperture in the intake tract. The plunger and valve member is biased to the closed position by a spring. When the solenoid is energised, this pulls the plunger into the solenoid coil against the bias of the spring, and the valve member will thus open the diversion aperture. When de-energised, the plunger and the valve member are returned by the spring to the closed condition. In this system the plunger is connected directly to valve member that opens and closes the diversion aperture and thus bypass path.
The OEM system operates as follows: the ECU monitors the accelerator pedal position, and if a rapid reduction in accelerator pedal position is detected, the ECU energises the solenoid coil to open the valve member and thus the bypass path. The ECU typically holds the bypass valve open for approximately 2 seconds, unless it detects that the accelerator pedal position is increased, at which point it will immediately de-energise the solenoid coil and thus close the diversion aperture and the bypass path.
There are three known versions of the factory diverter valve used: the first type uses a diaphragm and poppet-style valve that is connected to the solenoid plunger. There are holes in the face of the piston that transfer pressure to the back of the diaphragm in order to balance the opposing forces that result from the pressure acting an the areas in front of and behind the diaphragm, so that the sum of the forces is theoretically zero. This then means that when energised, the solenoid coil is able to pull the plunger, diaphragm and poppet valve open against the return spring, thereby opening up diversion aperture and the bypass path. When the solenoid is de-energised, the return spring returns the plunger, diaphragm and poppet valve to the closed position.
The second type use a plastic piston type valve that is connected to the solenoid plunger. Again, there are holes located in the face of the piston to equalise pressure front and back, thereby theoretically creating no resultant force. It operates in the same way as the diaphragm type described above.
The third type is the same as the second type, except it features a slotted “basket” that shrouds the piston. Its purpose can only be assumed to aid closure when the piston is open and the solenoid is then de-energised.
Deficiencies of factory-fitted bypass valves. These factory-fitted bypass valve types all share a common operating principle, which is primarily designed to eliminate pressure spikes and the associated noise. The ECU can only operate the solenoid in two states—on or off. Therefore, because the valve is directly connected to the solenoid plunger in all cases, the valve can only be open or shut. No method is provided for varying the opening size in response to the pressure in the intake tract, and as a result, turbo lag on these cars is less than optimised.
In addition, the evolution of the factory-fitted bypass valves indicates that there are inherent deficiencies in the design of the valve itself.
An inherent deficiency with the first type is that it s commonly known to fail even under normal operating conditions by rupturing of the diaphragm or tearing of the face seal on the piston. The second and third types seek to solve this problem of torn diaphragms, however the fact that the piston is made from plastic means that a close toleranced fit cannot be achieved between the piston and the sleeve. Since there is no diaphragm to seal this gap, a significant amount of air is able to leak from the rear chamber of the piston past the gap and into the recirculation path. This situation is less than ideal for performance, since pressurised air is being lost. Secondly, the piston type valve suffers a condition where it is unable to close, resulting in significant power loss. Once opened and when there is significant air being bypassed, if the throttle is re-opened very quickly and the solenoid de-energised, the piston is unable to close because of the weak return spring and the un-balanced forces on the piston caused by dynamic pressure of air rushing past the face of the piston—under these conditions there is a greater pressure acting on the face of the piston than there is on the back of the piston, causing it to be held open. The only way to get it to close again once this happens is to close the throttle once again until the bypassing air pressure drops enough to let the piston close.
The third type is a further evolution of the second type, with the vented “basket” a clear attempt to diffuse the bypass air to reduce its velocity so it can enter the transfer holes in the face of the piston in order to balance the forces and allow it to close. Unfortunately, this solution is only moderately effective, and certain conditions can still cause this valve type to be held open.
All of the above deficiencies of the three types of factory-fitted bypass valve are especially noticeable when the car has been modified to increase performance through higher boost pressure, as this is often associated with higher intake temperatures that can accelerate the rupture of the diaphragm type, and exacerbate the non-closure issue and leaking of the piston types.
After Market Valves: there are two common approaches taken by aftermarket bypass valve manufacturers to solve the problems of the factory-fitted bypass valves. Both methods involve replacing the entire factory-fitted solenoid coil and valve entirely with a pneumatically-operated valve. Where the two methods differ however is how the pneumatic valves are controlled.
One method supplies a 3-port solenoid valve that alternately connects a vacuum source and a pressure source to the pneumatic bypass valve—vacuum causes the bypass valve to open, pressure makes it close. The 3-port solenoid valve supplied with this kit connects to the factory wiring harness in the engine bay so it utilises the same signal from the ECU to determine when to open the bypass valve.
The second method does not use the ECU signal at all. Instead, it simply connects the bypass valve directly to the intake manifold, thereby causing the valve to open when the throttle is closed and the intake manifold is in vacuum, and when the throttle is open the intake manifold is pressurised and therefore the bypass valve is closed. This is the common method used by most after-market bypass valves.
Deficiencies of After-Market valves: despite both methods used in the after-market valves ensuring the deficiencies of the factory-fitted bypass valve are overcome (i.e. robustness, preventing boost pressure leaks, and the ability to close when required), these methods have their own set of deficiencies that cannot be overcome regardless of the design of the hardware.
The first method retains the ECU control, which is desirable for rapid response of the bypass valve opening, however to achieve this, the apparatus supplied in order to carry out this method is extensive, expensive, and time-consuming to install. The apparatus required to carry out the first method must include: a means to provide a hose connection to the intake manifold; a 3-port solenoid valve; a means of electrically connecting the 3 port solenoid valve to the factory wiring harness; a mounting bracket to hold the 3 port solenoid valve in place; sufficient lengths of vacuum hose to connect the 3 port solenoid valve; and a pneumatically-operated bypass valve. Installation of this apparatus requires significant time, typically at least an hour by an experienced vehicle mechanic.
Even once the above apparatus is installed, there is a key performance deficiency in reaction time. The addition of the 3 port solenoid valve and the associated lengths of vacuum hose connecting it to the bypass valve mean that there is a time delay between the ECU energising the 3 port solenoid valve and the bypass valve opening. This is further hindered by the fact that this apparatus still requires a vacuum before the bypass valve can begin to open—even when the 3 port solenoid has switched the source to the intake manifold, the throttle needs to be mostly closed before the intake manifold has sufficient vacuum to open the bypass valve. By this time a pressure spike may have already occurred.
The second method suffers similar deficiencies. The apparatus required to carry out the second method must include: a means to provide a hose connection to the intake manifold; a means to provide an electrical ballast load to the factory wiring harness to prevent the ECU from detecting a bypass valve fault since the factory-fitted solenoid coil has been removed; sufficient lengths of vacuum hose to connect the intake manifold valve to the bypass valve; a pneumatically-operated bypass valve. Like the first method, installation of this apparatus requires significant time, typically at least an hour by an experienced vehicle mechanic.
This method also suffers from delayed opening time for the same reason as the first method—the apparatus requires the intake manifold to achieve sufficient vacuum to aid bypass valve opening, by which time a pressure spike has already occurred.
Both methods require additional apparatus, which adds significant cost to the kit, complexity and time of installation, and therefore increased cost of installation. All of this comes at the expense of a delayed response time of the bypass valve, thereby failing to achieve one of the primary purposes of a bypass valve.
Any reference herein to known prior art does not, unless the contrary indication appears, constitute an admission that such prior art is commonly known by those skilled in the art about which the invention relates, at the priority date of this application.